Electric machines, e.g. motors and generators, have electrical conductors, a main insulation and a stator lamination stack. The main insulation serves the purpose of electrically insulating the conductors from one another, from the stator lamination stack and from the surroundings. During operation of the electric machine, sparks which can form “treeing” channels in the main insulation can occur as a result of electric partial discharges. The “treeing” channels can result in dielectric breakdown through the main insulation. A barrier against the partial discharges is achieved by the use of mica, which has a high partial discharge resistance, in the main insulation. The mica is used in the form of platelet-like mica particles having a conventional particle size of from a number of 100 microns to a number of millimeters, with the mica particles being processed to form a mica paper. To increase the strength and to improve the processability, an electrical insulation tape which has a support structure in addition to the mica paper is used.
To produce the main insulation, the electrical insulation tape is wound around the conductor. The electrical insulation tape is then impregnated with a synthetic resin and the synthetic resin is subsequently cured. To improve the partial discharge resistance of the main insulation, the use of nanosize particles which are dispersed in the synthetic resin before impregnation is known. However, the presence of the particles shortens the life of the synthetic resin. This shows up particularly in the form of progressive polymerization of the synthetic resin which leads to an increase in the viscosity of the synthetic resin and thus makes impregnation of the electrical insulation tape difficult.